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While infection control experts are grateful for a much-needed reprieve after the chaos of the H1N1 pandemic, they are scratching their heads and wondering what happened to the regular seasonal flu in North America.

Ontario typically sees 1,500 deaths associated with seasonal flu annually. This year it has seen none.

“There is nothing. There is no influenza at all right now except for a smattering of cases here and there,” said Dr. Michael Gardam, director of infectious disease control and prevention with the Ontario Agency for Health Protection and Promotion.

“It’s not at all clear where it’s gone,” he added.

This time of year is typically “high season” for seasonal flu, noted Dr. Kathleen Dooling, associate medical officer of health for Peel.

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“Compared to the fall of 2009, things are extremely quiet with respect to influenza and in comparison to past influenza seasons, things are very quiet,” she added.

Indeed, the province saw only one confirmed case of seasonal flu in the last week alone.

While influenza bugs have been under the microscope for decades, there is still much that is unknown about them.

“It’s quite remarkable how little we know about it despite having looked at it and worried about it. We are not even sure how it’s transmitted person-to-person,” Gardam said.

Theories about what has happened to seasonal flu this year abound. Has it been knocked out by the pandemic virus? Perhaps it is part of a cycle that sees influenza bugs lie low once a decade? Maybe all the extra vigilance over handwashing has kept it at bay. How about the mild winter?

“I can’t give you an explanation why, when H1N1 showed up, all the others vanished. We know that happens from other pandemics. When a new strain comes in, it tends to replace the other strains, but I can’t easily explain to you why that is,” Gardam said.

Dr. Allison McGeer, head of infection control at Mount Sinai Hospital, agreed it is possible that the old seasonal strains of Influenza A – H3N2 and another subtype of H1N1 — are being replaced by the pandemic H1N1 virus.

“But this doesn’t explain influenza B,” she said, referring to the third strain that makes up seasonal flu.

She also pointed out that seasonal flu is currently circulating in China.

“If you look at North America, it looks like seasonal flu is being replaced, but there is actually a fair amount of H3N2 activity in China so who knows about the North American perspective,” she said.

There is much speculation that next year’s seasonal flu will be made up of the pandemic H1N1 virus and influenza B virus.

But McGeer said it is impossible to predict with certainty because our experience with pandemics is so limited.

“We base our understanding of the world on natural experience. Well, we are basing our knowledge of pandemics on three of them. The same size is too small,” she said.

Because influenza viruses are so unpredictable, McGeer said she wouldn’t put money down betting what has happened to this year’s seasonal flu.

“Before every year, you don’t know how much activity there is going to be and you don’t know what the predominant strain is going to be,” she explained.

“On this list of possibilities is that it’s going to be a really quiet year and there is not going to be much of anything. Once every 10 years that’s what happens,” she said, noting that the last quiet flu season was in the winter of 2000-01.

Another theory floating around is that public campaigns urging people to wash their hands regularly and sneeze into their sleeves has stymied seasonal flu. But there is no evidence to back that one up.

“It’s hard to know as it hasn't been studied,” Gardam said.

Ontario’s former chief medical officer of health Dr. Richard Schabas, currently medical officer of health in Prince Edward and Hastings counties, doesn’t buy the theory that the mild winter is responsible for the lull in seasonal flu.

“If it were true that the severity of winter were important in influenza transmission, we would expect influenza transmission to be consistently worse the further north you go. But it isn’t.” he said.

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